This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c9cp00946a In situ reversible redox switching of first hyperpolarizability of bimetallic ruthenium complexes† C. Karthika, S. R. Sarath Kumar, L. Kathuria, P. K. Das * and A. G. Samuelson In this article we report the reversible redox switching of first hyperpolarizability of bimetallic ruthenium complexes bridged by bipyridyl tetrazine (bptz) ligands by second harmonic light scattering experiments (SHLS). We have synthesised [RuII(acac) 2 (CH 3 CN) 2 ] and [(acac) 2 Ru-bptz-Ru(acac) 2 ] complexes and measured their first hyperpolarizabilities as a function of in situ electrochemical oxidation/reduction of the metal centres. As a result of the oxidation of ruthenium centre from Ru(II) to Ru(III), the molecular hyperpolarizability of the complexes went up. The mixed-valence intermediate bimetallic complex and not the fully oxidized complex exhibit the highest b value of 780  10 30 esu. We also demonstrated that the hyperpolarizability is reversible with the change of the oxidation state of the metal and both the complexes are stable for several cycles of redox switching. The experimental results are also supported by calculations. 1 Introduction Compounds which show large second order nonlinear optical (NLO) response gained attention in the past few decades because of their wide range of applications in molecular electronics, optical computing and data processing. 1–5 The search for a better molecule among inorganic crystalline salts and conju- gated organic molecules led to the identification of molecules with large first hyperpolarizability (b). 6,7 However, inorganic salts suffer from easy structure manipulation ability and organic compounds from poor thermal, mechanical and photochemical stability. The effort to address these disadvantages led to organo- metallic compounds where metal atoms and organic moieties were brought together to stabilize the structure and enhance the molecular b as well as thermal, mechanical and photochemical properties of the molecules. The first such example of an organo- transition metal complex having large second order nonlinear optical properties reported by Green et al. 8 was a ferrocenyl compound having many times higher SHG efficiency than urea in the microcrystalline state. Subsequently, there have been several organometallic complexes reported in the literature with large values of b which, in bulk solids, manifest as high SHG efficiency. 9 Second order nonlinearity of a compound is dependent on its electronic structure and asymmetry of charge distribution over the molecular dimension and, therefore, b value of the compound can be manipulated by changing any of these factors. Generally one dimensional compounds having donor (D)–p–acceptor (A) architecture associated with intramolecular charge transfer transition exhibit large b values due to the enhanced anisotropic charge distribution. 10–13 Structural varia- tion in D–p–A molecules can be induced by a suitable external stimuli like change in pH, irradiation with suitable light or an external potential gradient 14–17 and lead to drastic changes in the b value. Compounds that exhibit switching of b value in a controlled and reversible manner with respect to an external stimuli can act as a molecular switch. 7,18–23 In this context, polarizable organometallic compounds with D–p–A architec- ture can be exploited to facilitate effective modification of electronic structure and thus high contrast in b values. 24–27 The first report on switching of b in a LB film came in 1991 from Sakaguchi et al., 28 where they had demonstrated photo- chemical switching of b of Ru(II)–bipyridine complexes. Later, Coe et al. 29 oxidized a series of Ru–bipyridyl complexes electro- chemically and followed the b value of the oxidised species by second harmonic light scattering and demonstrated that such redox active metal complexes can be explored for application as molecular switches. To enhance b and dimensionality of molecular switches Weyland et al. 30 synthesised multinuclear ferrocenyl complexes and measured their b values in different metal oxidation states. Asselberghs et al. carried out, in situ, reversible electrochemical switching of b of a substituted ferrocenyl complex and realized that the b value changes by a factor of 10 in going from Fe(III) to Fe(II) state of the metal. 31 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India. E-mail: pkdas@iisc.ac.in; Fax: +91 8023600416; Tel: +91 8022932662 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp00946a Received 17th February 2019, Accepted 7th May 2019 DOI: 10.1039/c9cp00946a rsc.li/pccp PCCP PAPER Published on 08 May 2019. Downloaded by Indian Institute of Science on 5/16/2019 4:29:00 PM. View Article Online View Journal